Adrenergic receptor (type of GPCR)

Adrenergic receptor (type of GPCR)

  • Class of metabotropic G protein coupled receptors that are targets of the catecholamines, especially norepinephrine (noradrenalin) and epinephrine (adrenaline).
  • Many cells possess these receptors, and the binding of a catecholamine to the receptor will generally stimulate the sympathetic nervous system.
  • Adrenergic receptors (“adrenergic” reflects the alternative name for epinephrine, adrenaline)

Two classes of adrenergic receptors: alpha adrenergic receptor and beta adrenergic receptor.

  • There are two types of α-adrenergic receptors, termed α1 (a Gq coupled receptor) and α2 (a Gi coupled receptor).
  • There are three types of β-adrenergic receptors, termed β1, β2 and β3 (All three are linked to Gs proteins).

α1 receptor activation:

  • α1 couples to Gq, which results in increased intracellular Ca2+.
  • Triggers smooth muscle contraction in blood vessels in the skin, gastrointestinal tract, kidney, and brain, among other areas.

α2 receptor activation:

  • Couples to Gi which causes a decrease of cAMP activity.
  • Triggers inhibition of insulin and the induction of glucagon release in the pancreas, contraction of GI tract sphincters, and increased thrombocyte aggregation.

β type receptors activation:

  • Couple to Gs, and increases intracellular cAMP activity.
  • Triggers heart muscle contraction, smooth muscle relaxation, and glycogenolysis.
Figure 1: Adrenergic signal transduction

Natural ligand for beta adrenergic receptor: epinephrine hormone

Agonists are structural analogs that bind to a receptor and mimic the effects of its natural ligand.

  • The hormone that initiates the signaling pathway is called a first messenger, which activates a second messenger in the cytoplasm
  • Amino acid derived hormones and polypeptide hormones are not lipid-derived (lipid-soluble) and therefore cannot diffuse through the plasma membrane of cells.
  • Lipid insoluble hormones bind to receptors on the outer surface of the plasma membrane, via plasma membrane hormone receptors.

Example: Epinephrine, norepinephrine etc

Figure 2: The 2D structure of A) noradrenaline (norepinephrine) B) adrenaline (epinephrine)

β adrenergic receptors cause changes in energy metabolism such as:

  • an increase in glycogen breakdown in muscle and liver cells.
  • an increase in triglyceride breakdown (lipolysis) in adipose tissue.

The β-Adrenergic Receptor System Acts through the Second Messenger cAMP

Transduction of the epinephrine signal: the β adrenergic pathway (steps involved)

Figure 3: Transduction of the epinephrine signal: the β adrenergic pathway.

Step 1:  Binding of epinephrine to the receptor

  • Epinephrine action begins when the hormone binds to a protein receptor in the plasma membrane of a hormone sensitive cell.
  • Promotes a conformational change in the receptor’s intracellular domain.
  • The activated receptor then interacts with the heterotrimeric GTP-binding stimulatory G protein (Gs) on the cytosolic side of the plasma membrane.

Step 2: Activation of G protein coupled to the receptor

  • Binding of epinephrine enables the receptor to catalyze displacement of bound GDP by GTP, converting Gs to its active form.

G protein bound to GDP: Inactive

G protein bound to GTP: Active

Step 3: Activation of Adenylyl cyclase enzyme

  • When the nucleotide binding site of Gs is occupied by GTP, Gs is active and can activate adenylyl cyclase.

Adenylyl cyclase (Effector enzyme)

Location: An integral protein of the plasma membrane, with its active site on the cytosolic face.

Function: It catalyzes the synthesis of cAMP from ATP.

How they are activated: The association of active Gαs with adenylyl cyclase stimulates the cyclase to catalyze cAMP synthesis raising the cytosolic [cAMP].

  • This stimulation by Gαs is self-limiting. Gαs is a GTPase that turns itself off by converting its bound GTP to GDP and inactive Gαs dissociates from adenylylcyclase, rendering the cyclase inactive.
  • After Gαs reassociates with the β and ϒ subunits reforming the heterotrimer. Gs is again available to interact with a hormone-bound receptor
Figure 4: Self-inactivation of Gs

The protein’s intrinsic GTPase activity, in many cases stimulated by RGS proteins (regulators of G protein signaling) determines how quickly bound GTP is hydrolyzed to GDP and thus how long the G protein remains active.

Step 4: Synthesis of cAMP by Adenylyl cyclase enzyme using ATP

Figure 5: Synthesis of cAMP by Adenylyl cyclase enzyme using ATP

cAMP has a very short half-life:

  • It is hydrolyzed to AMP, that has no second messenger activity.
  • The reaction is catalyzed by cyclic nucleotide phosphodiesterase

Caffeine and theophylline ( methylxanthines contained in coffee and tea  respectively), inhibit the phosphodiesterase, thereby increasing the half-life of cAMP, and enhancing its effects.

Step 5: cAMP activates protein kinase A (PKA)

  • Protein kinase Ais a family of enzymes whose activity is dependent on cellular levels of cyclic AMP (cAMP).

In the absence of cAMP:  Tetrameric R2C2 complex is catalytically inactive

  • The inactive form of PKA contains two catalytic subunits (C) and two regulatory subunits (R)
  • An auto inhibitory domain of each R subunit occupies the substrate binding site of each C subunit. The regulatory subunits mask the catalytic domains and prevent kinase activity.

In the presence of cAMP: PKA active with only catalytic units no regulatory units

  • When cAMP binds to two sites on each R subunit, the R subunits undergo a conformational change and the R2C2 complex dissociates to yield two free catalytically active C subunits.
  • Unmasking of the catalytic subunits leads to the activation of enzyme PKA

The catalytic subunit of PKA contains: The active site and a domain to bind regulatory subunit.

The regulatory subunit contains: Domains to bind to cyclic AMP, a domain that interacts with catalytic subunit and an auto inhibitory domain.

Below is a list of the steps involved in PKA activation:

  1. Cytosolic cAMP increases
  2. Two cAMP molecules bind to each PKA regulatory subunit (Four cAMP molecules are able to bind to the two R subunits)
  3. The regulatory subunits move out of the active sites of the catalytic subunits and the R2C2 complex dissociates
  4. The free catalytic subunits interact with proteins to phosphorylate Ser or Thr residues.

Catalysis

The liberated catalytic subunits can then catalyze the transfer of ATP terminal phosphates. to protein substrates at serine, or threonine residues. This phosphorylation usually results in a change in activity of the substrate.

PKA phosphorylates Ser/Thr when present in this sequence:

X – Arg – (Arg/Lys) – X – (Ser/ Thr)

Figure 6: Activation of cAMP dependent protein kinase (PKA).

Step 6: Phosphorylation of cellular proteins by PKA and cellular response to the ligand

Target protein of PKA in glycogenolysis (In liver): Phosphorylase b kinase.

One of the target proteins of the phosphorylase kinase is glycogen Phosphorylase b.

  • It catalyses the phosphorylation of two specific Ser residues in inactive Phosphorylase b and convert it into active glycogen phosphorylase a which degrade glycogen.
  • The enzyme protein phosphatase Ӏ can dephosphorylate active phosphorylase a back into inactive Phosphorylase b form.
Figure7: Epinephrine signaling cascade showing the phosphorylation of target enzymes by PKA

 Phosphorylase a catalyses the following reaction:

Glycogen (n glucose residues) + Pi Glucose 1-Phosphate + Glycogen (n-1 glucose residues)

Finally group enzymes complete the glycogenolysis. (Details later)

  • glucose-1-phosphate → glucose-6-phosphate
  • glucose-6-phosphate + H2O → glucose + Pi
Figure 8: Regulation of glycogen metabolism by cAMP in liver and muscle cells.

Figure 8 (a): an increase in cytosolic cAMP activates PKA, which inhibits glycogen synthesis directly and promotes glycogen degradation via a protein kinase cascade. At high cAMP, PKA also phosphorylates an inhibitor of phosphoprotein phosphatase (PP). Binding of the phosphorylated inhibitor to PP prevents this phosphatase from dephosphorylating the activated enzymes in the kinase cascade or the inactive glycogen synthase.

Figure 8 (b): Decrease in cAMP inactivates PKA, leading to release of the active form of phosphoprotein phosphatase. The action of this enzyme promotes glycogen synthesis and inhibits glycogen degradation.

Amplification of signal

  • The effect of a hormone is amplified as the signaling pathway progresses.
  • The binding of a hormone at a single receptor causes the activation of many G-proteins, which activates adenylyl cyclase.
  • Each molecule of adenylyl cyclase then triggers the formation of many molecules of cAMP.
  • The second messenger cAMP now activates PKA, each molecule of which catalyzes the phosphorylation of many molecules of the target protein phosphorylase b kinase
  • This kinase activates glycogen phosphorylase b, which leads to the rapid mobilization of glucose from glycogen.
  • In this way, a small amount of hormone can trigger the formation of a large amount of cellular product (The net effect of the cascade is amplification of the hormonal signal by several orders of magnitude)
  • To stop hormone activity, cAMP is deactivated by the cytoplasmic enzyme phosphodiesterase, or PDE. PDE is always present in the cell and breaks down cAMP to control hormone activity, preventing overproduction of cellular products.
Figure 9: Epinephrine cascade.
Figure 10: Simple chart showing amplification of an external signal downstream from a cell surface receptor

The β-Adrenergic Receptor Is Desensitized by Phosphorylation

  • Signal transducing systems undergo desensitization when the signal persists.
  • Desensitization of the beta adrenergic receptor is mediated by a protein kinase that phosphorylates the receptor on the intracellular domain that normally interacts with Gs
  • When the receptor is occupied by epinephrine, β adrenergic receptor kinase (βARK) phosphorylates Ser residues near the carboxyl terminus of the receptor.
  • The phosphorylation creates a binding site for the protein β arrestin (βarr).
  • βARK is normally located in the cytosol and is drawn to the plasma membrane by its association with the Gsβϒ
  • Binding of β arrestin effectively prevents interaction between the receptor and the G protein.
  • The binding of β arrestin also facilitates receptor sequestration (the removal of receptors from the plasma membrane) by endocytosis into small intracellular vesicles.
  • Receptors in the endocytic vesicles are dephosphorylated and then returned to the plasma membrane.
  • Beta Adrenergic receptor kinase is a member of a family of G protein coupled receptor kinases (GRKs).
  • They phosphorylate serpentine receptors on their carboxyl-terminal cytosolic domains and play roles similar to that of βARK in desensitization and resensitization of their receptors.

At least five different GRKs and four different arrestins are encoded in the human genome; each GRK is capable of desensitizing a subset of the serpentine receptors, and each arrestin can interact with many different types of phosphorylated receptors.

Figure11: Desensitization of the beta adrenergic receptor in the continued presence of epinephrine (mediated by βARK and β arrestin)

 

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